1. Field of the Invention
The invention relates to a method for controlling the degree of polymerization of alkali cellulose and to a method for producing cellulose ether for use in the fields of chemistry, medicine, etc., based on the control method.
2. Description of the Related Art
Proposed methods for reducing the viscosity of cellulose ether include a method of using hydrogen peroxide (Japanese Patent Application Examined Publication (JP-B) No. 48-19232), a method of using a volatile acid such as hydrogen halide (Japanese Patent Application Examined Publication (JP-B) No. 48-26385), a method of using ozone (Japanese Patent Application Unexamined Publication (JP-A) No. 55-145701), and a method of using gamma rays or electron beams (Japanese Patent Application Examined Publication (JP-B) Nos. 47-3964 and 47-3965).
Unfortunately, these methods are not industrially satisfactory because the reagent for use in reducing viscosity is expensive, the reagent remains in the product, or a special device is necessary.
Therefore, Japanese Patent Application Unexamined Publication (JP-A) No. 61-264001 proposes a method without these problems, which includes controlling the amount of oxygen in a reactor at the start of reaction in the process of producing alkali cellulose. The degree of polymerization of cellulose is reduced by reaction (depolymerization reaction) between oxygen and cellulose in the presence of alkali Accordingly, as the amount of oxygen increases, the viscosity of the resulting cellulose ether decreases. Since oxygen in the air can be used, this method is inexpensive and needs no special apparatus, and oxygen does not remain in the product.
According to Japanese Patent Application Unexamined Publication (JP-A) No 59-56401, after a powdery pulp is added to a reactor having an internal stirring structure, the oxygen in the reactor is first adjusted to an amount necessary for the desired viscosity, and subsequently stopped from entering the reactor to produce alkali cellulose. As the pulp comes into contact with the alkali, the pulp reacts with oxygen in the reactor so that it is depolymerized.
Japanese Patent No. 4087534 discloses a method of producing alkali cellulose, comprising the steps of: reducing the amount of oxygen in a reactor to 1 g or less per kilogram of cellulose before the addition of an alkali; and supplying oxygen with an amount required for obtaining the desired viscosity to the reactor during or after the addition of an alkali.
R. I. C. Michie, S. M. Neale: J. Polymer Sci., A2, 2063 (1964) discloses a method comprising the steps of placing a dispersion of cellulose in an aqueous sodium hydroxide solution in a vessel, allowing the cellulose to absorb oxygen at a constant temperature in the closed vessel, and measuring the pressure in the vessel to determine the absorbed amount of oxygen, which is 0.360 to 0.985 ml per gram of cellulose.